Generally, some image forming systems include a sheet post-processing apparatus that performs post-processing on sheets. For example, the sheet post-processing apparatus supports a plurality of sheets stacked on a processing tray. An ejector is disposed on an upstream side of the processing tray in a sheet transport direction. The ejector supports the plurality of sheets on the processing tray. The ejector is fixed to an ejector belt. The ejector belt is rotated by a stepping motor or the like. When the ejector belt rotates, the ejector moves the plurality of sheets to a downstream side of the processing side.
In order to move the plurality of sheets to the downstream side, a bundle hook is used together with the ejector. The bundle hook is fixed to a bundle hook belt. When the bundle hook belt rotates, the sheets supported by the ejector are delivered to the bundle hook. The bundle hook transports the sheets downstream of the processing tray.
In order to return the ejector from the downstream side to an original position on the upstream side, a winding spring is used. By a stepping motor, the ejector belt rotates and the winding spring is extended. When the winding spring is extended, elastic energy is stored in the winding spring. When interlocking of the stepping motor and the winding spring is released, the winding spring returns the ejector to the original position. That is, the elastic energy stored in the winding spring is used for returning the ejector to the original position.